Converting an integer to binary in C

If you want to transform a number into another number (not number to string of characters), and you can do with a small range (0 to 1023 for implementations with 32-bit integers), you don't need to add char* to the solution

unsigned int_to_int(unsigned k) {
    if (k == 0) return 0;
    if (k == 1) return 1;                       /* optional */
    return (k % 2) + 10 * int_to_int(k / 2);
}

HalosGhost suggested to compact the code into a single line

unsigned int int_to_int(unsigned int k) {
    return (k == 0 || k == 1 ? k : ((k % 2) + 10 * int_to_int(k / 2)));
}

You need to initialise bin, e.g.

bin = malloc(1);
bin[0] = '\0';

or use calloc:

bin = calloc(1, 1);

You also have a bug here:

 bin = (char *)realloc(bin, sizeof(char) * (sizeof(bin)+1));

this needs to be:

 bin = (char *)realloc(bin, sizeof(char) * (strlen(bin)+1));

(i.e. use strlen, not sizeof).

And you should increase the size before calling strcat.

And you're not freeing bin, so you have a memory leak.

And you need to convert 0, 1 to '0', '1'.

And you can't strcat a char to a string.

So apart from that, it's close, but the code should probably be more like this (warning, untested !):

int int_to_bin(int k)
{
   char *bin;
   int tmp;

   bin = calloc(1, 1);
   while (k > 0)
   {
      bin = realloc(bin, strlen(bin) + 2);
      bin[strlen(bin) - 1] = (k % 2) + '0';
      bin[strlen(bin)] = '\0';
      k = k / 2;
   }
   tmp = atoi(bin);
   free(bin);
   return tmp;
}

Just use itoa to convert to a string, then use atoi to convert back to decimal.

unsigned int_to_int(unsigned int k) {
    char buffer[65]; /* any number higher than sizeof(unsigned int)*bits_per_byte(8) */
    return atoi( itoa(k, buffer, 2) );
}